FIG. 1 illustrates a general method for manufacturing a high pressure discharge lamp for a projector.
In Step S100, a bulb is prepared. In this bulb preparation step, a light emitting part of the bulb made of fused quartz is filled with argon, halogen, mercury, or the like and sealed. Sealing parts are formed at both ends of the light emitting part. Electrodes made of tungsten are placed in the light emitting part, and molybdenum foils are attached to the electrodes. Moreover, if necessary, the sealing parts are engraved with a lot number by laser marking.
In Step S110, an initial driving is performed for several minutes to several hours. This initial driving step is intended for aging, screening, and the like.
In Step S120, a lead wire and, if necessary, a trigger wire are attached to the bulb.
In Step S130, the bulb is attached to a reflector. This attachment step includes a step of mounting the bulb onto the reflector, a step of positioning the bulb relative to the reflector, and a step of fixedly attaching the two. In the positioning step, generally an operation is performed in which the bulb in a driving state is positioned at an optimum position relative to the reflector.
In Step S140, when necessary, inspection driving is performed. Here, checked are: whether the illuminance and the like meet the specification, whether the optical axis is not shifted, and so forth.
After Step S140, a high pressure discharge lamp thus completed is incorporated into a projector main body in Step S200. Thus, a projector is completed.
As the usage mode of projectors, there are a desktop type and a ceiling type. As the ceiling type, a projector of the desktop type simply turned upside down can be used, and an image is merely inverted.
Conventionally, the use of the desktop type has been common. However, in the recent years, the ceiling type is increasingly used for education. For this reason, a projector has often been used in such a manner that the projector is turned upside down depending on the initial driving and the actual use driving.
In such a circumstance, we have repeatedly manufactured and tested prototypes. As a result, we have found that there is a difference in time when devitrification occurs in a lamp between the desktop type and the ceiling type. As a result of the analysis, devitrification is assumed to have a mechanism as follows.
Note that, in this description, “driving” means driving in the horizontal direction.
(1) A surface modified layer or a burr formed during preparation of an electrode of a bulb is left on the electrode surface.
(2) Within approximately several seconds after the start of an initial driving of the bulb, the burr or the like on the electrode surface is dispersed together with mercury in the bulb and adheres to an inner wall thereof.
(3) In an evaporation process of mercury with increase in the temperature of the bulb, a difference in temperature occurs between an upper portion and a lower portion of the bulb. Specifically, the temperature of the upper portion is high, while the temperature of the lower portion is low.
(4) Within approximately several tens of seconds after the start of the initial driving, mercury and a tungsten residue on a hotter upper portion of the inner wall are removed.
(5) Within approximately several tens of seconds after the start of the initial driving, the temperature of the entire bulb is increased, mercury on a lower portion of the inner wall also evaporates, but the tungsten residue is left unremoved.
(6) After the initial driving is completed, mercury adheres to the electrode and the vicinity thereof, while the tungsten residue left on the lower portion of the inner wall stays adhering to the lower portion of the inner wall.
(7) If the bulb is driven for actual use in the same direction as that in the initial driving, devitrification hardly occurs. Meanwhile, if the actual-use driving is performed with the bulb upside down, the originally lowermost portion on which the tungsten residue remains is then placed at an uppermost portion. As a result, this tungsten-residue-adhering portion is located at the highest-temperature position when a lamp is driven.
(8) Accordingly, devitrification occurs as pure quartz changes to cristobalite with the tungsten-residue-adhering portion serving as the core.
In normal use for the desktop type, the up-down direction coincides between the initial driving and the actual-use driving. The tungsten-adhering portion is located at the lower portion (low-temperature portion). Thus, the problem of devitrification rarely occurs.
However, the introduction of the ceiling type hinders the up-down direction for the actual use from being specified in the manufacturing of a high pressure discharge lamp or a projector for this type. For this reason, a measure to prevent devitrification is needed, even if a high pressure discharge lamp manufactured by a single manufacturing method is used for the desktop type or for the ceiling type.
As one method for dealing with this devitrification problem, for example, Patent Document 1 discloses a technique including an electronic ballast and a rotary motor. The motor rotates a bulb or a high pressure discharge lamp, if necessary (for example, the motor rotates the component by a predetermined angle for each event such as turning a light on or off). Thereby, a specific portion of the bulb is not fixed to an uppermost portion, that is, the highest-temperature portion.
In this manner, a portion to be the upper portion of the bulb is changed upon each event to avoid fixation of the tungsten-residue-adhering portion to the high-temperature portion. Thus, occurrence of devitrification is prevented.
Prior Art Document List
Patent Document
Patent Document 1: JP Patent Publication 2007-48736